function fourColorsFrame(ctx, width, height, text) {
  const kYellow = "#FFFF00";
  const kRed = "#FF0000";
  const kBlue = "#0000FF";
  const kGreen = "#00FF00";

  ctx.fillStyle = kYellow;
  ctx.fillRect(0, 0, width / 2, height / 2);

  ctx.fillStyle = kRed;
  ctx.fillRect(width / 2, 0, width / 2, height / 2);

  ctx.fillStyle = kBlue;
  ctx.fillRect(0, height / 2, width / 2, height / 2);

  ctx.fillStyle = kGreen;
  ctx.fillRect(width / 2, height / 2, width / 2, height / 2);

  ctx.fillStyle = 'white';
  ctx.font = (height / 10) + 'px sans-serif';
  ctx.fillText(text, width / 2, height / 2);
}

// Paints |count| black dots on the |ctx|, so their presence can be validated
// later. This is an analog of the most basic bar code.
function putBlackDots(ctx, width, height, count) {
  ctx.fillStyle = 'black';
  const dot_size = 10;
  const step = dot_size * 3;

  for (let i = 1; i <= count; i++) {
    let x = i * step;
    let y = step * (x / width + 1);
    x %= width;
    ctx.fillRect(x, y, dot_size, dot_size);
  }
}

// Validates that frame has |count| black dots in predefined places.
function validateBlackDots(frame, count) {
  const width = frame.displayWidth;
  const height = frame.displayHeight;
  let cnv = new OffscreenCanvas(width, height);
  var ctx = cnv.getContext('2d');
  ctx.drawImage(frame, 0, 0);
  const dot_size = 10;
  const step = dot_size * 3;

  for (let i = 1; i <= count; i++) {
    let x = i * step + dot_size / 2;
    let y = step * (x / width + 1) + dot_size / 2;
    x %= width;
    let rgba = ctx.getImageData(x, y, 1, 1).data;
    const tolerance = 40;
    if (rgba[0] > tolerance || rgba[1] > tolerance || rgba[2] > tolerance) {
      // The dot is too bright to be a black dot.
      return false;
    }
  }
  return true;
}

function createFrame(width, height, ts) {
  let text = ts.toString();
  let cnv = new OffscreenCanvas(width, height);
  var ctx = cnv.getContext('2d');
  fourColorsFrame(ctx, width, height, text);
  putBlackDots(ctx, width, height, ts);
  return new VideoFrame(cnv, { timestamp: ts });
}